The dual seal ring arrangement in FIG. 4 of the Mahoff patent advantageously provides a redundant seal whereby should one of the seal rings fail, the other will maintain a seal between the coupling members. Mahoff's redundant seal ring arrangement, however, has the undesirable effect of reducing the interference which is required between each seal ring and the coupling members for establishing a tight seal.
In the Mahoff coupling, the seal rings are torsionally deflected to their seal-establishing conditions by engaging the seal rings with formations on the coupling members and by axially drawing the coupling members together. Upon being torsionally deflected, each seal ring will be deformed into a cone-shaped configuration in which the seal ring's radial dimension is increased, thereby creating the required interference or sealing pressure for establishing the fluid-tight seal.
Because of the arrangement of the Mahoff seal rings with the coupling members, the radial forces exerted by the seal rings on each coupling member will be in the same direction. These seal ring-exerted forces will therefore be cumulative and will have the undesirable effect of radially deflecting the seal ring-engaging portions of the coupling members away from each other to reduce the tightness of the seal between the coupling members.
The effect of the cumulative radial forces on the coupling members in the Mahoff coupling is normally not a significant problem in small couplings because of the relatively high radial stiffness of the coupling members. However, in large couplings having tube sizes greater than three inches, the radial stiffness of the coupling members is reduced to such an extent that it is insufficient to prevent the coupling members from being radially deflected to a substantial degree under the influence of the cumulative seal ring-exerted forces. For large coupling sizes, the sealing pressure is therefore reduced to a level that is often too low for establishing a satisfactory seal.
The present invention overcomes the foregoing problem but retains the advantage of the dual seal ring redundancy.